1. Field of the Invention
The present invention relates to a process for the modification with liquid additives of water-redispersible dispersion powder compositions based on polymers of ethylenically unsaturated compounds.
2. The Prior Art
Water-redispersible dispersion powder compositions based on polymers of ethylenically unsaturated compounds are becoming increasingly important for a very wide variety of applications. The starting point for the preparation of such redispersible dispersion powders comprises the corresponding aqueous dispersions of the polymers, which are dried by means of known processes, usually with the addition of spraying aids and antiblocking agents. Drying is generally carried out by means of spray drying, with freeze-drying and, in certain cases, thin-film drying of the polymer dispersions also being possible. Processes for preparing redispersible dispersion powders are described in numerous prior patents, such as EP-A 78,449 and EP-A 149,098, which corresponds to U.S. Pat. No. 4,859,751.
To adapt the properties of the dispersion powders to the requirements of the individual applications it is often necessary to modify the dispersion powders with additives. In this context, the subsequent addition in particular of liquid additives, for instance hydrophobicizing agents, emulsifiers, adhesion promoters, crosslinking agents, biocides or film-forming auxiliaries, presents problems, since their addition frequently leads to blocking of the powders, or else the required quantity cannot be mixed in homogeneously.
EP-A 493,168 proposes the spraying, together with the polymer dispersion, of the liquid polysiloxanes employed as hydrophobicizing agents. A disadvantage of this procedure is that, in the case of co-spraying, in some cases the alkoxysilane functions of polysiloxanes can hydrolyze prematurely under the conditions of spray drying. Silanes and siloxanes with groups which hydrolyze with relative ease, for example oxime, amine and acyl groups, cannot be incorporated in this way.
The modification of dispersion powders by co-spraying the liquid additives with the polymer dispersion is also not capable of universal application in other cases. In the case of the co-spraying of numerous emulsifiers with polymer dispersions it is possible, due to interactions of the emulsifiers or else of water-soluble, liquid additives with the protective colloids which are employed as spraying aid, for the glass transition temperature thereof to be reduced to such an extent that blocking of the polymer particles takes place. This has an adverse effect on both the redispersibility of the powders and their storage stability.
Similarly, low-boiling or steam-volatile, liquid additives cannot be incorporated by co-spraying, since under the spray-drying conditions they escape, at least in part, together with the steam or inert gas. This would make it necessary to procure and operate an expensive recovery plant or a residue incineration plant for incineration of the often expensive liquid vapors which pass into the waste gas.
In the case of mixing in additives by means of co-spraying, great difficulties are caused in particular by liquid additives which are soluble in water or autoemulsifiable in water. The conceivable alternative, of mixing in such liquids to the powder subsequently, leads to the problems already mentioned above of blocking and/or inhomogeneity of the mixture.